Process control plants or systems often employ rotary valves such as, for example, butterfly valves to control the flow of process fluids. In general, rotary valves typically include a fluid flow control member disposed in a fluid flow path between an inlet and an outlet of the rotary valve. The fluid flow control member is rotatably coupled relative to a valve body of the rotary valve via a shaft. Typically, a portion of the shaft extending from the rotary valve is operatively coupled to an actuator (e.g., a pneumatic actuator, a hydraulic actuator, etc.) that rotates the valve shaft in a first direction and a second direction opposite the first direction.
In operation, a control unit (e.g. a positioner) may supply a control fluid (e.g., air) to the actuator to position the fluid flow control member to a desired position to regulate or throttle the flow of fluid through the rotary valve. The actuator may move the fluid flow control member through a complete stroke between a fully open position to allow fluid flow through the valve and a fully closed position to prevent fluid flow through the valve.
Rotary valve actuators such as rack and pinion type actuators are often used to implement, for example, quarter-turn rotary valves. A rack and pinion type actuator converts rectilinear motion of two opposing pistons into rotational motion of the valve shaft. Rack and pinion type actuators often provide a relatively high output torque and a compact physical envelope or footprint. However, rack and pinion actuators may require a damping mechanism to provide a cushioning and/or deceleration effect to prevent noise, shock and/or damage to the components of the actuator.
However, some known damping mechanisms used with rotary actuators dampen the movement of the actuator over an entire stroke of the actuator. For example, such known damping mechanisms may dampen or reduce the speed of the pistons over an entire closing stroke. Such a configuration significantly reduces the efficiency of the actuator and significantly reduces or limits the overall closing speed and output torque provided by the actuator.